This invention relates to a radio receiver, and in particular to a method of demodulating received signals.
Many radio transmission systems rely on frequency modulation. That is, information is encoded in the phase or frequency of a carrier signal.
One known way of decoding the information in a radio receiver is to apply a frequency conversion to the received signal, to remove the carrier signal, leaving the baseband modulated signal. However, this method has the disadvantage that the modulated signal which is detected may include offset components, and it is difficult to eliminate these offsets, without also eliminating a part of the wanted signal.
As a result of the problems which arise when dealing with signals at baseband, it is more common to convert received signals to an intermediate frequency for demodulation.
As mentioned above, there are problems associated with conventional methods of demodulating signals at baseband. However, the present invention is concerned with attempting to solve these problems, by allowing the elimination of constant disturbance components of the signals. The ability to achieve accurate demodulation of baseband components is of particular advantage in receivers which are intended to process two or more different types of signals, because various components can then be used to process both types of signal.
In order to be able to eliminate constant disturbance components of the baseband signals, it is first necessary to be able to detect such components.
In some cases, it will be known that the mean values of the undistorted components of the signal should be zero, in which case the actual mean values can be determined to be the distortion components. In other cases, the detected signal can be subject to an initial processing so that, in the absence of distortion, it should have a zero mean, in which case, again, any actual mean value can be taken to be a distortion component. In other situations, relationships between two components of the baseband signal can be used to determine whether there is any constant disturbance component.